1. Field of the Invention
The present invention relates to flow control valves of the ball or segmented ball type and more particularly to such valves having equal percentage characteristics, i.e. a characteristic wherein a percentage change in ball travel causes an equal percentage change in flow volume at all points on the characteristic curve of the valve.
2. Description of the Prior Art
A ball valve that exhibits an equal percentage characteristic has a number of useful control applications including the control of the recirculating cooling water in nuclear reactors. Heretofore, the recirculating cooling water in a nuclear reactor was controlled using a pair of variable speed pump motor-generator sets to provide particular flow rates and volumes under varying conditions of time, temperature and power demand. Notwithstanding the great expense involved in providing variable speed pump motor-generator sets, it has been found that the accuracy of response of such devices may not be as finely controlled as desired.
With the advent of newer and larger nuclear reactors the pressures and volumes of fluid flow required for the recirculation of cooling water in such reactors necessitated pumps which were extremely large and very expensive.
It was proposed that a ball valve be used to replace the variable speed pump motor-generator sets for providing control of cooling water flow; however, it was discovered that ball valves and segmented ball valves experience a sudden increase in flow rate at some point beyond the three-quarters open position of the valve and in particular in the 89 - 91% of maximum full open range. Thus, use of ball valves in precise control applications has been limited because of the poor control provided and the liklihood of system oscillation. The ball valve was totally unacceptable for use in nuclear power plants since surges in cooling water flow cannot be tolerated in large nuclear reactors.
The ball valve was only acceptable for use in control systems if the ball travel was limited to 80 percent of the maximum available travel. Thus, the valve had to be 25 percent oversized to produce a desired capacity resulting in a substantial increase in cost.